Apparatus for growing single crystals and purifying substances



July 10, 1956 A. T. HORTON 2,754,180

APPARATUS FOR GROWING SINGLE CRYSTALS AND PURIFYING SUBSTANCES FiledDec. 30, 1952 2 Sheets-Sheet l INVEN TOR.

JGENT July 10, 1956 A. T. HORTON 2,754,180

APPARATUS FOR GROWING smcuz CRYSTALS AND PURIFYING SUBSTANCES Filed Dec.50, 1952 2 Sheets-Sheet 2 INVENTOR.

fiver Ivrorfon BY A6ENT tea APPARATUS FOR GRQWHQG SINGLE CRYSTALS AND PGSUBSTANCES Avery T. Horton, Raleigh, N. (3., assignor to the UnitedStates of America as represented by the Secretary of Commerce Theinvention described herein may be manufactured and used by or for theGovernment of the United States for governmental purposes without thepayment to me of any royalty thereon in accordance with the provisionsof the act of March 3, 1883, as amended (45 Stat. 467; 35 U. S. C. 45).

The present invention relates to an apparatus for growing singlecrystals and for purifying substances, and in particular to an apparatuswhich provides a plane temperature boundary at the interface of twoimmiscible liquids which are maintained at different temperatures.

It was well known in the prior art that single crystals of a substancecould be produced by passing the substance which was to be formed into acrystal from a first temperature zone above the melting point of thesubstance through a plane temperature boundary into a second temperaturezone below the melting point of the substance under consideration. Itwas also well known that as growth the crystal proceeded the substancewas puri fied by rejection of less pure material. One of the prior artmethods of producing this plane temperature boundary between twodifferent temperature zones consisted of two combustion furnacesseparated by a thin layer of a heat insulating material which regulatedthe temperature boundary between the two zones. The heat insulatinglayer had an opening which nearly adhered to the walls of thecrystallization tube containing the substance to be crystallized, whichtube was withdrawn from the upper to the lower furnace by means of aplatform elevator. There are several disadvantages to this method, suchas, an inferior temperature boundary and nonuniform temperatures in theheated zones due to circulation of air between the two furnaces. Thislatter disadvantage was particularly serious in that it caused a dilfusezone of temperature on either side of the boundary which producedhorizontal temperature gradient. This often made for the nonuniformhorizontal formation of the crystals and thereby caused stresses in thefinal product. Also, there is a large time lag in obtaining equilibriumtemperatures, which is due to the low heat capacity of the air in thesystem. In this prior art apparatus it was not possible to observe theprocess during the growth of. the crystal, and this also made itimpossible to remelt a selected portion of the sample to remove therejected impurities.

The object of the present invention is to provide an ap-.

paratus for growing single crystals in which temperature equilibrium maybe quickly established and easily maintained.

Another object of the invention is to provide an apparatus for growingsingle crystals in which uniform temperatures may be maintained in theheated zones.

Another object of the invention is to provide an apparatus for growingsingle crystals in which there is a nondiffusc zone of temperaturetransition.

Another object of the invention is to provide an apparatus for growingsingle crystals in which the temperatures in the two zones may be variedover wide P: 2,754,133 ififi Patented July 10, 1953 ranges withoutappreciably affecting the quality of the temperature boundary.

Another object of the invention is to provide an apparatus for growingsingle crystals in which the passage of the crystallization tube fromone zone to the other does not appreciably affect the temperatureboundary.

Another object of the invention is to provide an apparatus in whichselected portions of the material may be remelted as desired.

Another object of the invention is to provide between the twotemperature zones a stable plane temperature boundary having a very hightemperature gradient.

Another object of the invention is to provide a system in which there isa minimum of heat exchange between the two temperature zones.

In accordance with the present invention there is provided a columnwhich contains two immiscible liquids heated to different temperaturesthereby producing two uniform temperature zones and a plane temperatureboundary at their interface. Resistance wire is wound around the twosections of the column to heat the respective liquids to the desireddifierent temperatures. The

level of the interface in the column is automatically maintained at apredetermined height. A motor is used to lower the crystallization tubethrough the temperature interface at a constant rate. Thecrystallization tube is suspended from a Wire and is weighted andcentered by a weight attached to the bottom.

Other uses and advantages of the invention will become apparent uponreference to the specification and drawings.

Figure 1 is a drawing of the overall apparatus of the present invention.

Figure 2 is a detailed view of the crystallization tube.

Figure 3 is a drawing of a modification of the apparatus showingre-entrant thermal wells for more accurately measuring the temperaturesin the two zones.

Referring to Figure 1, there is shown a glass column 1 containing twoimmiscible liquids. The first immiscible liquid is contained in theupper portion of the glass column above the plane temperature boundary 2which is represented by the dotted line. The second immiscible liquid islocated below this plane temperature boundary. The column is providedwith an arm 3 which permits overflow of the upper liquid into therecycling reservoir 4. The glass tube 6 is inserted into the reservoirand connected to the check valve 7. The check valve 7 is connected toanother check valve 8 by the tubing 9. Connected into the tubing 9between the two check valves is another tube 11 which is connected to apump 12 which consists of a piston 13 riding in a cylinder 14. Thepiston is operated by a motor 16. The liquid flowing through the checkvalve 8 is fed back into the column 1 through the tubing 17 and coldwater condenser 18. The bottom end of the cold water condenser passesthrough a hole in the stopper 19 in the top of the column 1. It will benoted that the tube 17 is offset from the center of the cold watercondenser so that the cable 21 which supports the crystallization tube22 may pass through the center of the center column of the cold watercondenser. The cable 21 is attached to an essentially constant speedmotor 23, which motor controls the speed at which the crystallizationtube is lowered through the column 1. Suspended from the bottom of thecrystallization tube 22 is the drive weight and guide 24 which is usedto maintain the crystallization tube erect and to keep the cable 21taut. The bottom of the column 1 is connected through the U-shaped tube26 and the rubher tubing 27 to the leveling bulb 28. The arm 29 of theleveling bulb directs the overflow from the bulb into the reservoir 31.The upper portion of the column 1 above the plane temperature boundary 2is wound with resistance wire 3%) which is connected to a variablesource of voltage, not shown, for maintaining the upper liquid at apredetermined temperature. The lower portion of the column below theplane temperature boundary is also wound with resistance wire 35 whichis used to maintain this portion of the column at a predeterminedtemperature.

In operation the two immiscible liquid layers are brought to the desiredtemperature above and below the freezing point of the substance inquestion. The interface between the two liquids is adjusted to thedesired height in the column by lowering or raising the leveling bulb28. The sample, previously sealed in the crystallization tube 22 underits own vapor pressure, is attached to the lowering mechanism and therecycling system for the upper layer is started. The combination of theleveling bulb 23 and the recirculation system maintains the temperatureboundary at a constant height during the operation. There are many otherwell known methods of keepi'n'gthe boundary at a constant height and thepresent method forms no part of the presen't invention. In someinstances it may be desirable to allow the level to rise or fall inwhich case the heating element's'are raised or lowered also.

The sample in the crystallization tube is allowed to melt completely andthen lowered to a position in the column so that the bottom portion ofthe tube 22, which has a capillary tube, is just above the liquidinterface. The motor 23 is started and the crystallization tube isslowly lowered through the temperature interface. In some cases it mightbe advantageous to reverse the temperatures of the two zones and toraise the tube from the lower to the upper level. After completecrystallization (that is, when the sample is in the lower liquid) thesample is slowly brought to room temperature to prevent fractures of thecrystal after growth.

The apparatus provides many advantages over that of the prior art, sinceit is possible to maintain a very sharp temperature boundary between thetwo immiscible liquids and to maintain nondiffuse temperature zones oneither side of the interface. This apparatus is very flexible in thatthe temperature of each immiscible layer can be independently variedover very wide limits and still permit the maintenance of a goodtemperature boundary condition at the interface. This was one of themain disadvantages of the prior art system where there was continuitybetween the air in. the two furnaces which readily permitted conductionof heat from'one furnace to the other. These advantages stem from thefact that the heat contact within each liquid is excellent while heatcontact between the two different liquids is very poor. Also the presentapparatus provides for the passage of the crystallization tube from onetemperature zone into the other without destroying the boundarycondition. This is made possible by using immiscible liquids which willrepel each other, and therefore permits a minimum of intermingling andheat exchange. Of very great importance is the fact that the sharptemperature boundary permits r'emelting of a selected portion of thecrystal, which is very useful in separating the crystal from therejectedi'mpurities.

Figure 2 shows a cross section of the crystallization tube which is usedin the practice of the present invention. The crystallization tube hasthe general shape of an elongated hexagon to which is connected acapillary tube 32 having a ring in'the bottom to which the drive weightand guide are attached. To the upper end of the hexagon is attached acolumn 33 that is necked down and then formed into a series of bulbsconnected by'necked-down portions. The purpose for this is as follows:It -is .apparent that during crystallization and/or purification of asubstance by the method just described, the impurities in the materialinitially placed in the crystallization tube will' be expelled duringthe formation of a crystal and will rise to the top of the mass. Theseimpurities will gather in the V1 region. Thereafter, the V1 portion isremelted, using a reverse heating system in which the lower liquid is atthe higher temperature, and suspending the tube upside down in thecolumn. The very sharp plane temperature boundary makes it possible toselect with great accuracy the exact portion which will be remelted. Thematerial that had been in the V1 section will now run down into thelowermost portion of the tube V1 and, after solidification, this partcan be sealed off from the rest of the system and removed. Thenthesample tube minus this lowermost section is reintroduced into thesystem, the sample remelted and crystallized as before. The rejectedimpurities from the second run reside in the portion V2 and are removedin the manner just described. This can be carried on for as long as itis practical or desirable, depending upon the degree of purity required.

Figure 3 shows a crosssection of another embodiment of the inventionshowing only the main column 1. 'In this modification the center portion36 of the column 1 is increased in diameter to allow the re-entranttubes 37 to be formed in the column. These tubes permit the temperaturein the column to be measured at various points to insure a uniformtemperature throughout the upper and lower liquids, respectively, beforethe system is placed in operation.

It will be apparent that the embodiments shown are only exemplary andthat various modifications can be made in construction and arrangementWithin the scope of my invention as defined in the appended claims.

I claim:

'1. An apparatus for growing crystals of a substance, comprising ahollow vertical shell, means for maintaining a column of liquids in saidshell, two immiscible liquids in said shell constituting said column ofliquids producing a plane temperature boundary at the interface of saidliquids, means for separately heating said liquids to differenttemperatures respectively, one of said liquids being heated above themelting point of the substance to be crystallized and the second of saidliquids being heated to a point below the melting point of thesubstance, a crystallization tube positioned within said shell andimmersed in said liquids for holding the substance to be crystallizedand means for withdrawing said tube from-the first liquid into thesecond liquid.

2. The invention of claim 1 in which said crystallization tube isprovided with at least one impurity collection chamber having a neckportion adapted to be sealed off to enable removal of said collectionchamber from the crystallization tube.

3. An apparatus for growing crystals of a substance, comprising a hollowvertical shell, means for maintaining a column of liquids in said shell,two immiscible liquids in said shell constituting said column of liquidsproducing a plane temperature boundary at the interface of said liquids,means for separately heating said liquids to different temperaturesrespectively, one of said liquids being heated above the melting pointof the substance to be crystallized and the second of said liquids beingheated to a point below the melting point of the substance, acrystallization tube positioned Within said shell and immersed in saidliquids for holding the substance to be crystallized and means forwithdrawing said tube from the first liquid into the second liquid at apredetermined rate, and means for maintaining the interface at apredetermined height.

4. An apparatus for growing single crystals of a substance comprising afirst container, a first and a second liquid located in said container,said liquids being immiscible and producing a plane boundary at theinterface of said liquids, means for maintaining said first liquid abovethe melting point of the substance to be crystallized and means formaintaining said second liquid below the melting-point of saidsubstance, a second crystallization container in which said substance islocated H $754,186 7 V I 5 6 positioned within said first container andimmersed in References Cited in the file of this patent said liquids andmeans for Withdrawing said second con- UNITED STATES PATENTS tainer fromsaid first liquid nto said second liquid at a 1,560,473 Howard Nov. 3,1925 umfom" predate rate 5 1,793,672 Bridgman Feb. 24, 1931 1,879,445Othmer Sept. 27, 1932 2,449,664 Marisic Sept. 21, 1948

1. AN APPARATUS FOR GROWING CRYSTALS OF A SUBSTANCE, COMPRISING A HOLLOWVERTICAL SHELL, MEANS FOR MAINTAINING A COLUMN OF LIQUIDS IN SAID SHELL,TWO IMMISCIBLE LIQUIDS IN SAID SHELL CONSTITUTING SAID COLUMN OF LIQUIDSPRODUCING A PLANE TEMPERATURE BOUNDARY AT THE INTERFACE OF SAID LIQUIDS,MEANS FOR SEPARATELY HEATING SAID LIQUIDS TO DIFFERENT TEMPERATURESRESPECTIVELY, ONE OF SAID LIQUIDS BEING HEATED ABOVE THE MELTING POINTOF THE SUBSTANCE TO BE CRYSTALLIZED AND THE SECOND OF SAID LIQUIDS BEINGHEATED TO A POINT BELOW THE MELTING POINT OF THE SUBSTANCE, ACRYSTALLIZATION TUBE POSITIONED WITHIN SAID SHELL AND IMMERSED IN SAIDLIQUIDS FOR HOLDING THE SUBSTANCE TO BE CRYSTALLIZED AND MEANS FORWITHDRAWING SAID TUBE FROM THE FIRST LIQUID INTO THE SECOND LIQUID.